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Mitochondrial DNA
Mitochondrial DNA (mtDNA) is the DNA located in organelles called mitochondria. Most other DNA present in eukaryotic organisms is found in the cell nucleus. Nuclear and mitochondrial DNA are thought to be of separate evolutionary origin, with the mtDNA being derived from the circular genomes of the bacteria that were engulfed by the early ancestors of today's eukaryotic cells. Each mitochondrion is estimated to contain 2-10 mtDNA copies. In the cells of extant organisms, the vast majority of the proteins present in the mitochondria (numbering approximately 1500 different types in mammals) are coded for by nuclear DNA, but the genes for some of them, if not most, are thought to have originally been of bacterial origin, having since been transferred to the eukaryotic nucleus during evolution. Among multicellular animals (metazoans), nearly all of the mtDNA in a fertilized egg (zygote) is inherited from only one parent - the female. One mechanism for this is simple dilution: an egg contains 100,000 to 1,000,000 mitochondria, whereas a sperm contains only 10 to 100. Another mechanism, documented for a few organisms, is that the sperm mitochondria do not enter the egg. Whatever the mechanism, this single parent (uniparental) pattern of mtDNA inheritance is found in most animals, most plants and in fungi as well. In humans (and probably in metazoans in general), 100-10,000 separate copies of mtDNA are usually present per cell (egg and sperm cells are exceptions). In mammals, each circular mtDNA molecule consists of 15,000-17,000 base pairs, which encode the same 37 genes: 13 for proteins (polypeptides), 22 for transfer RNA (tRNA) and one each for the small and large subunits of ribosomal RNA (rRNA). This pattern is also seen among most metazoans, although in some cases one or more of the 37 genes is absent and the mtDNA size range is greater. Even greater variation in mtDNA gene content and size exists among fungi and plants, although there appears to be a core subset of genes that are present in all eukaryotes (except for the few that have no mitochondria at all). Some plant species have enormous mtDNAs (as many as 2,500,000 base pairs per mtDNA molecule!) but, surprisingly, even those huge mtDNAs contain the same number and kinds of genes as related plants with much smaller mtDNAs. Use in identification Unlike nuclear DNA, which is inherited from both parents and in which genes are rearranged in the process of recombination, there is usually no change in mtDNA from parent to offspring. Although mtDNA also recombines, it does so with copies of itself within the same mitochondrion. Because of this and because the mutation rate of animal mtDNA is higher than that of nuclear DNA , mtDNA is a powerful tool for tracking ancestry through females (matrilineage) and has been used in this role to track the ancestry of many species back hundreds of generations. Human mtDNA can be used to identify individuals. Because the base sequence of animal mtDNA changes rapidly, it is useful for assessing genetic relationships of individuals or groups within a species and also for identifying and quantifying the phylogeny (evolutionary relationships; see phylogenetics) among different species, provided they are not too distantly related. To do this, biologists determine and then compare the mtDNA sequences from different individuals or species. Data from the comparisons is used to construct a network of relationships among the sequences, which provides an estimate of the relationships among the individuals or species from which the mtDNAs were taken. This approach has limits that are imposed by the rate of mtDNA sequence change. In animals, the rapid rate of change makes mtDNA most useful for comparisons of individuals within species and for comparisons of species that are closely or moderately-closely related, among which the number of sequence differences can be easily counted. As the species become more distantly related, the number of sequence differences becomes very large; changes begin to accumulate on changes until an accurate count becomes impossible. Mitochondrial inheritance Female inheritance In sexually reproducing organisms, mitochondria are normally inherited exclusively from the mother. The mitochondria in mammalian sperm are usually destroyed by the egg cell after fertilization. Also, most mitochondria are present at the base of the sperm's tail, which is used for propelling the sperm cells. Sometimes the tail is lost during fertilization. In 1999 it was reported that paternal sperm mitochondria (containing mtDNA) are marked with ubiquitin to select them for later destruction inside the embryo. Paternally inherited mitochondria have also been reported in some insects such as the fruit flyKondo R, Matsuura ET, Chigusa SI (1992). Further observation of paternal transmission of Drosophila mitochondrial DNA by PCR selective amplification method. Genet Res 59: 81-84. and the honeybee.Meusel MS, Moritz RF (1993). Transfer of paternal mitochondrial DNA during fertilization of honeybee (Apis mellifera L.) eggs. Curr Genet 24: 539-543. Evidence supports rare instances of male mitochondrial inheritance in some mammals as well. Specifically, documented occurrences exist for mice,Gyllensten U, Wharton D, Josefsson A (1991). Paternal inheritance of mitochondrial DNA in mice. Nature 352: 255-257. Shitara H, Hayashi JI, Takahama S, Kaneda H, Yonekawa H (1998). Maternal inheritance of mouse mtDNA in interspecific hybrids: segregation of the leaked paternal mtDNA followed by the prevention of subsequent paternal leakage. Genetics 148: 851-857. where it was subsequently rejected. It has also been found in sheep,Zhao X, et al. (2004). Further evidence for paternal inheritance of mitochondrial DNA in the sheep (Ovis aries). Heredity 93:399-403. and in cloned cattle.Steinborn R, Zakhartchenko V, Jelyazkov J, Klein D, Wolf E, Muller M et al (1998). Composition of parental mitochondrial DNA in cloned bovine embryos. FEBS Lett 426: 352-356. It has been found in a single case in a human male and was linked to infertilitySchwartz M, Vissing J (2002). Paternal inheritance of mitochondrial DNA. N Engl J Med 22: 576-580.. While many of these cases involve cloned embryos or subsequent rejection of the paternal mitochondria, others document in vivo inheritance and persistence under lab conditions. Genetic influence Genetic illness Mutations of mitochondrial DNA can lead to a number of illnesses including exercise intolerance and Kearns-Sayre syndrome (KSS), which causes a person to lose full function of their heart, eye, and muscle movements. (See also Mitochondrial disease). See also * Mitochondrial disease * Human mitochondrial genetics * Paternal mtDNA transmission * Single origin theory * Mitochondrial Eve * Mitochondrial CRS References External links * mtDNA testing at DNA Heritage * Mitomap - a human mitochondrial genome database http://www.mitomap.org/ * A polymorphism in mitochondrial DNA associated with IQ? * mtDNA sequencing information * mtDNA and the global diaspora of modern humans Professor Stephen Oppenheimer's Genetic Map * Mitosearch (FTDNA) * EMPOP - Mitochondrial DNA Control Region Database Category:DNA